1. Field of the Invention
This invention relates to brushless DC motors, and in particular to a brushless DC motor which uses a composite magnet.
2. Description of the Related Art
U.S. Pat. No. 4,851,731, issued to Saotome et al., discloses a flat type DC motor in which vibrations of the rotor axle are prevented by providing an asymmetrical magnetic field between the stator yoke and the rotor. The disc-shaped rotor magnet comprises a solid piece on which magnetic north and south poles are alternatingly arranged. However, because opposite magnetic poles are formed in the same material (described in a later section regarding the "Block" wall effect), Saotome et al. provide less than optimal definition of poles. Moreover, magnetizing a solid material typically entails winding a wire around the solid material in a predetermined manner and then applying voltage thereto. This method of magnetizing frequently fails to provide total saturation of the material, thereby failing to provide optimal field strength.
U.S. Pat. No. 4,187,441, issued to Oney on Feb. 5, 1990, discloses a permanent magnet rotor disk comprising a plurality of permanent magnet sections secured by spokes. As noted explicitly in Oney, the spokes are constructed of non-magnetic material such as aluminum or an aluminum alloy. Thus, although Oney has the capability to provide total saturation of the permanent magnet sections, Oney actually introduces a large pole transition area with the non-magnetic spokes. Moreover, securing individual permanent magnet sections between the spokes yields a highly labor-intensive process, thereby significantly increasing manufacturing cost.
Therefore, a need arises for a brushless DC motor having both a high power output density as well as a high power output per unit cost.